1. Field of the Invention
The present invention relates to an electric double layer capacitor apparatus including a plurality of electric double layer capacitors and a method for adjusting voltages of the plurality of electric double layer capacitors.
2. Description of the Background
Power supplies which include a plurality of electric double layer capacitors are disclosed in U.S. Pat. No. 5,783,928 (hereinafter referred to as the "'928 patent"), entitled "Storage Capacitor Power Supply"; U.S. Pat. No. 5,982,050 (hereinafter referred to as the "'050 patent"), entitled "Power Supply Unit For Automotive Vehicle"; U.S. Pat. No. 5,932,932 (hereinafter referred to as the "'932 patent"), entitled "Storage battery voltage control apparatus"; and Japanese Unexamined Patent Publication (Kokai) 10-201091 (hereinafter referred to as the "'091 publication"), entitled "Power Supply Unit For Automotive Vehicle Utilizing Electric Double Layer Capacitors." The contents of these applications are incorporated herein by reference in their entirety.
In an electric double layer capacitor apparatus disclosed in the '928 patent, referring to FIG. 13, an electric double layer capacitor apparatus includes a plurality of electric double layer capacitors 1 (hereinafter referred to as "a cell" or "cells"). Each cell 1 has a current bypass circuit 16 which is connected to each cell 1 in parallel. An operation voltage of the shunt regulator 7 is set at a maximum charge voltage (V.sub.u) of the cell 1 by adjusting a ratio of resistance of the resistors (13 and 15). When the voltage between a positive and negative pole terminals (3A and 3B) of the cell 1 is equal to the maximum charge voltage (V.sub.u), the shunt regulator 7 turns on and therefore current flows to the resistor 5. Accordingly, a PNP transistor 9 turns on and prevents all capacitors from being applied with voltages exceeding maximum charge voltage. When the voltage between the positive and negative pole terminals (3A and 3B) is equal to the maximum charge voltage (V.sub.u).
In FIG. 13, it is supposed that capacitance (C1) of the cell (1a) is 1000 (F), capacitance (C2) of the cell (1b) is 1150 (F) and charge current is constant at 10 (A). Further, initial voltages of the cells (1a and 1b) are equal to zero (V) and the maximum charge voltage (V.sub.u) is set at 2.5 (V) and equal to a uniform voltage.
Referring to FIG. 14, the voltage of the cell (1a) which has smaller capacitance becomes the maximum charge voltage (V.sub.H) at (t1) before the voltage of the cell (1b) becomes the maximum charge voltage (V.sub.H). Then, the voltage of the cell (1b) becomes the maximum charge voltage (V.sub.H) at (t2) which is after (t1) by (.DELTA.t). (.DELTA.t) is approximately calculated as follows: EQU .DELTA.t=(C2-C1)*V.sub.H /I=37.5 seconds.
Heat of about 25 (W) generates in the bypass circuit (16) when bypass current flows for the period of .DELTA.t.
The '050 patent discloses a power supply unit which includes a plurality of electric double layer capacitors and a plurality of current bypass circuits which are connected to each cell in parallel, respectively. Each circuit has a Zener diode. The voltage of each cell is substantially adjusted to be Zener voltage.
The '091 reference discloses a power source which includes a plurality of electric double layer capacitors and a plurality of discharge circuits in order to adjust the voltage of each cell. Each discharge circuit is connected to each cell in parallel via a switch. When an accessary switch of a car is turned on, each switch connects each discharge circuit to each cell. Accordingly, the voltage of each cell is adjusted. Each switch may be manually turned on to discharge each cell after an engine stops.
The '932 reference discloses a storage battery voltage control apparatus which includes a plurality of electric double layer capacitors and a voltage-correcting storage battery over switches and a current-limiting resistor. In order to adjust the voltage of each cell, the voltages of each of the storage battery cells high in voltage can be equalized by transferring the charge to a storage battery cell low in voltage.